Lecture 3

EN 318

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EN 318

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Status Register

• The STATUS register contains

– Arithmetic status of the ALU

– RESET status

– Bank select bit for data memory.

• As with any register, the STATUS register can

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• As with any register, the STATUS register can

be the destination for any instruction.

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Status Register

• Z, DC, C, TO and PD bits are not writable.

– These bits are set or cleared according to device logic.

• For example, CLRF STATUS will clear the upper-three bits and set the Z bit. This leaves the

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upper-three bits and set the Z bit. This leaves the STATUS register as 000u u1uu (where u = unchanged).

• Only the BCF, BSF, SWAPF and MOVWF instructions should be used to alter the STATUS register because these instructions do not affect any status bit.

PIC Instruction Set

• The instruction set for the PIC16F84

consists of 35 single word instructions

• These are grouped into three major

categories

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categories

– Byte-oriented

– Bit-oriented

– Literal and control

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List and Equates

• Before writing your program, you need to describe the

type of device that the program is to be burned to

List p=16F84

• Equate

– Similar to variable declaration in other programming

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languages.

– Labels are assigned to addresses. Later, whenever that label is

referred to in the program, the compiler looks up its address.

• E.g. Assigns ‘portB’ to the file register located at 0x06.

Port B is always located at this file register.

portB equ 0x06

Sample Program

LIST P=16F84

PORTB equ 0x06

TRISB equ 0x06

STATUS equ 0x03

RP0 equ 0x05

org 0

movlw 0x0FD

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movlw 0x0FD

movwf PORTB

bsf STATUS, RP0

bcf TRISB 0x080, 1

bcf TRISB 0x080, 2

movlw 0x07F

• Each instruction is a 14-bit word divided into:

• An OPCODE which specifies the instruction type

• One or more operands which further specify the operation of the

instruction.

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Field Description

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CLRF

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CLRW

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ADDLW

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ADDWF

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MOVLW

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MOVF

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MOVWF

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Exercise

• Write a program to clear W and Port B, then add

0x0A to 0x0B and send it out thru Port B.

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CALL

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GOTO

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Example for CALL and GOTOlist p=16F84

goto Main

; Subroutine

Delay

movlw 0xff

movwf 0x0c

L1 nop

decfsz 0x0c,1

goto L1

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goto L1

return

; Main program

Main

movlw 0x0

tris porta

tris portb

movlw 0xff

movwf portb

call Delay

Exercise

• Write a subroutine to clear PortB and send data

in W to PortB

• Write a program to add 0xA to 0xB and send it to

PortB (use the subroutine)

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INCF

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DECF

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INCFSZ

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DECFSZ

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NOP

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Exercise

• Write a subroutine to delay for 1 msec, given that

your input clock is 4 MHz

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• Loops & Timing analysis

• Subroutine

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